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Optimizing and Applying Graphene As a Saturable Absorber For Optimizing and Applying Graphene as a Saturable Absorber For Generating Ultrashort Pulses by Jonah Maxwell Miller A thesis submitted to the faculty of the University of Colorado in partial fulfillment of the requirements for the award of departmental honors in the Department of Physics 2011 This thesis entitled: Optimizing and Applying Graphene as a Saturable Absorber For Generating Ultrashort Pulses written by Jonah Maxwell Miller has been approved for the Department of Physics Thomas Schibli John Cumalat Jeanne Clelland Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Miller, Jonah Maxwell (BA Physics) Optimizing and Applying Graphene as a Saturable Absorber For Generating Ultrashort Pulses Thesis directed by Professor Thomas Schibli Over the last decade, a variety of exciting applications have been found for lasers that generate ultra- short pulses of light with durations of just a few femtoseconds, known as femtosecond lasers (fs-lasers) [1]. People now routinely measure optical frequencies [2, 3], atomic and molecular spectra, lengths, distances [4], and displacements [5] with fs-lasers, and new applications are constantly being discovered. Pulses of such short duration can be achieved from passively mode-locked lasers|that is, lasers in which the longitudinal electromagnetic waves in the laser cavity, or \modes," are locked into phase with each other [6, 7, 8, 9]. To lock the phase of the modes, a saturable absorber|a device which absorbs some percentage of low-intensity light, but which allows high-intensity light to pass through with reduced absorption|is used [6, 7, 8, 9]. To produce short pulse-width, high repetition-rate (many pulses per second) lasers, a saturable ab- sorber that becomes opaque quickly after being \saturated" by light and that saturates very easily is needed [6]. In this work, the potential for single atomic-layer graphene|a honeycomb lattice of carbon atoms only one atom thick, which has already proven itself to be an extraordinary material [10, 11, 12, 13, 14, 15, 16, 17]| as a saturable absorber is explored, and a method for producing high-quality graphene saturable absorbers is developed. This high-quality graphene's nonlinear (saturable) absorption was probed optically by differential transmission and pump-probe measurements and the possibility of tuning graphene's optical properties by chemical doping is explored by Raman spectroscopy and compared to doping concentration and measure- ments made in differential transmission and spectrophotometry. It is concluded that while graphene could be a highly desirable saturable absorber, it is currently limited by its relatively high saturation fluence com- pared to its damage threshold. The possibility of a graphene-based high-speed electro-optic modulator is also briefly discussed. This work is a step in the development of graphene as a saturable absorber comparable to but substantially cheaper than semiconductor saturable absorber mirrors (SESAMs), and towards the development of graphene-based optical and electro-optical devices for lasers. To Alexandra Fresch Without your emotional support throughout my college career, or your presence as a role model, I might never have pursued an honors thesis. v Acknowledgements I would like to thank the Principal Investigator, Professor Thomas Schibli, for his patience, guidance and support throughout this project. I owe a great deal also to Professor Schibli's student, Chien-Chung Lee, with whom I have worked very closely. He has shared with me a great deal of his knowledge of optics, lasers, and semiconductor physics. He guided me through the construction of the differential transmission setup and helped me first learn my way around an optics lab, performed all of the pump-probe measurements on graphene, built the ultra-low pressure CVD furnace we use to grow our samples, doped the graphene I measured in Raman, measured its Fermi Level by spectrophotometer, and measured its nonlinear absorption by the differential transmission setup we built together. I would also like to thank the other members of Professor Schibli's group, both past and present, for their support and for many helpful discussions, including: Brian Benton, David Miller, Wanyan Xie, Seiya Suzuki, Linna Cooley, and Jeffrey Hart. I would also like to express profound gratitude towards Doctor Kaoru Minoshima, AIST/NMIJ Tsukuba, Japan, who provided the Schibli group with an Er:Yb:glass gain medium, and from the Uni- versity of Colorado at Boulder: Professor Markus Raschke for lending me the use of his micro-Raman setup, Professor John Cumalat for the diamond sample, and Samuel Berweger and Doctor Joanna Atkin for lending me their expertise in Raman spectroscopy. This research was supported in part by the NNIN at the Colorado Nanofabrication Laboratory, The National Science Foundation under Grant No. ECS-0335765, and the Innovative Seed Grant Program and the Undergraduate Research Opportunities Fund (UROP) at the University of Colorado at Boulder. vi Finally, I would like to thank my parents and my grandparents, whose continued love, encourage- ment, support, and belief in my abilities have made it possible for me to go from a very mediocre student to an honors student, and who have always encouraged me to ask as many questions about the world as possible. Thank you, everyone; this project would not have been possible without you. Contents Chapter 1 Introduction 1 1.1 Motivation . .1 1.1.1 Mode-Locked Lasers: Their Potential and Their Challenges . .1 1.1.2 Graphene: The Wonder Material . .2 1.1.3 Graphene and Mode-Locked Lasers . .3 1.2 Overview . .4 2 General Background 5 2.1 The Mechanics of Mode-Locking in Brief . .5 2.2 The Physics of Semiconductor Saturable Absorbers . .9 2.2.1 Band Structure . .9 2.2.2 The Interaction Between Light and Matter . 11 2.2.3 The Dynamics of Saturable Absorption . 15 2.3 Graphene . 22 2.3.1 Band Structure . 22 2.3.2 Ultrafast Properties . 24 3 Methods for Measuring the Saturable Absorption of Graphene 27 3.1 Experimental Overview . 27 3.2 General Experimental Considerations . 28 viii 3.2.1 The Laser . 28 3.2.2 Lock-in Amplifiers . 31 3.3 Differential Transmission . 32 3.3.1 The Goals and Theory of Differential Transmission . 32 3.3.2 Implementation . 36 3.4 Time-Resolved Spectroscopy . 40 4 Optimizing Graphene Growth and Transfer Methods 43 4.1 A Brief Overview of Methods to Produce Graphene . 43 4.2 Methods . 45 4.3 Recipe Case Studies . 46 4.4 Optical Damage . 55 5 Characterization of Doped Graphene 56 5.1 Doping and State Blocking . 56 5.2 Spectrophotometry and Differential Transmisson of Doped Graphene . 58 5.2.1 Spectrophotometry . 58 5.2.2 Differential Transmission . 60 5.3 Raman Spectroscopy . 60 5.3.1 Theory of Raman Scattering . 62 5.3.2 The Raman Spectrum of Graphene . 72 5.3.3 Previous Work . 77 5.3.4 Our Study . 80 6 Conclusions And Outlook 89 Appendix A Final Graphene Growth Recipes and Transfer Method 92 ix A.1 Transfer Method . 92 A.2 Selected Recipes for Graphene Sheets and for Large Domain-Size Graphene Flakes. 94 A.2.1 Recipe for Graphene Sheets . 94 A.2.2 Recipe for Large Domain-Size Graphene Flakes . 94 B Calibration of the Home-Built Raman System 96 B.1 Y-Axis Calibration . 96 B.2 X-Axis Calibration . 101 C Python Scripts Used to Analyze Raman Data 105 C.1 rayleigh.py: The Offset Calculator . 106 C.2 RamanRecalibration.py: Batch Data Preparation For The Holographic Grating . 109 C.3 raman fs.py: Batch Data Preparation for the 600BLZ Grating . 115 C.4 LorentzFit.py: Batch-Fitting Curves to Raman Spectra . 121 D Conference Poster and Awards 127 E Paper Submitted Regarding the Doping of Graphene by Nitric Acid 134 Bibliography 143 Figures Figure 2.1 Electromagnetic Modes in a Laser Cavity . .5 2.2 Pulse Narrowing Through Saturable Absorption . .6 2.3 Examples of Band Structure . .9 2.4 The Different Conduction Phases of a Semiconductor . 12 2.5 Absorption, Stimulated Emission, and Saturable Absorption . 14 2.6 Lattice and Band Structure of Graphene . 22 2.7 Saturable Absorption of Graphene Near the Dirac Point . 26 2.8 Saturable Absorption of Graphene as a Function of Time . 26 3.1 Er:Yb:Glass Laser . 30 3.2 Differential Reflectivity for a Slow SESAM . 33 3.3 The Differential Measurement Technique . 34 3.4 The Differential Transmission System Used to Study Graphene . 35 3.5 Nonlinearity in Our Differential Transmission System . 38 3.6 Sample Differential Transmission Measurements . 40 3.7 Time-Resolved Spectroscopy System . 41 4.1 One Recipe Attempted During the Optimization Process . 47 4.2 Saturable Absorption of Low-Pressure Graphene . 49 4.3 Ultra-Low Pressure Vacuum System . 50 xi 4.4 Comparison of Graphene Grown by ULP One- and Two-Step Processes . 51 4.5 Graphene Grown with Extremely Low Partial Pressures of Methane . 54 5.1 State Blocking in Graphene . 56 5.2 Hole-Doping of Graphene . 57 5.3 The Tunable Optical Properties of Graphene . 61 5.4 A Toy Model of a Molecule . 62 5.5 Definition of Coordinate System for Classical Raman Scattering . 63 5.6 Raman Active and Inactive Modes . 66 5.7 The Quantum Description of Scattering . 68 5.8 Resonant Stokes Scattering . ..
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